You can't ask for much better than Stiller, Black and Downey Jr. in this offbeat and seriously funny comedy (note it finally kicked the Dark Knight out of the #1 spot). Tom Cruise's cameo as slimy movie exec Les Grossman is priceless.
Downey Jr. manages to steal the show in a tour-de-force role (acutally, 3 roles perhaps?) as bad-boy Aussie ultra-Method actor (Russel Crowe, anyone?), who goes ultra-character in his African American role, then outwits some bad guys by playing a Japanese samurai type for a too-brief yet funny scene. Stiller and Black are great as usual, as are the rest of the supporting cast.
Great opening "commercials" and "trailers" open the film, and tie directly in to the central characters. Funny parody of Hollywood with awesome actors and some great cameos. I can't wait to own this one so I can watch it again! (the gag reel should be good..)